Such structures are required to operate in precision structural components by giving off heat through radiation. These structures operate primarily in a pure gaseous environment or in a vacuum.
In connection with rocket propulsion systems for operation in outer space, for example in connection with position control drive means, radiation cooling is the preferred type of cooling. Compared to the regenerative cooling, the radiation cooling has several advantages. For example, radiation cooling does not require any complicated hollow wall structures with cooling channels. Further, radiation cooling does not involve any power losses, for example, due to driving of cooling medium pumps. Such pumps and power consumption are necessary for the regenerative cooling. Further, due to material strength considerations, it is necessary to keep the temperature of the propulsion system walls definitely below the melting temperature of these walls. Thus, high propulsion power outputs combined with high heat flow densities are possible only by using corrosion resistant materials having a high melting point. Such materials, for example platinum, however have a very low emission coefficient, or rather heat emission coefficient so that the advantage of the higher permissible wall temperature is again lost totally, or at least partially.
The application of layers or coatings having a better radiation emission characteristic is generally defeated due to the lack of a sufficient bonding strength and temperature resistance of such layers relative to the wall structure.
German Patent Publication (DE-OS) No. 3,446,243 discloses a microstructure for improving the heat removal by radiation or even heat absorption of a structural component surface. The known microstructure comprises straight ribs alternating with grooves having a rectangular groove bottom. The ribs themselves have a rounded facing outer edge. The grooves are filled with a material that is permeable to infrared radiation for increasing the mechanical strength of such wall structures and to also improve the corrosion resistance thereof. Glass or synthetic material is suitable for this filling purpose. The prior art does not make any statements with regard to the precise dimensions of the ribs and/or grooves, nor is there any mention regarding the production of such grooves. Generally, however, it is known that a rectangular groove bottom results in notching stresses, whereby the structural component is mechanically weakened. Further, the combination of the grooves with a filler material does not permit high structural component temperatures. This is so on the one hand due to a limited temperature stability of the filling material such as synthetic material and, on the other hand, it is due to a limited or insufficient bonding strength between the filler material and the groove walls. It is believed that the limited bonding strength is caused by the different heat expansion due to different heat expansion coefficients of the wall material and the filler material.
U.S. Re. Pat. No.: 30,077 describes microstructure surfaces for improving the bubble formation in nucleate boiling of liquids. The bubble formation is improved by providing the interface surface between the solid and the liquid with narrow deep grooves. The ribs remaining between these grooves are locally deformed in their ridge zone so that the groove width becomes smaller toward the exit end of the groove. The largest groove density corresponds to nine grooves per millimeter. The minimal groove depth corresponds to 203 .mu.m. The smallest groove width in the exit zone is 13 .mu.m. These conventional grooves for the purpose of nucleate boiling are cut by conventional tools, for example, by milling or shaping on a shaping machine or by deformation without cutting, for example by a rolling operation. The enlarged scale micrographs of polished sections as, for example shown in FIGS. 2, 3, and 5, illustrate the very large variations in the groove shape, position, and dimensions so that definite statements of any kind must be considered to be merely rough mean values. Hence, the production method disclosed in U.S. Pat. No. Re. 30,007 is not usable for applications where the surface structure and thus, for example, the radiation characteristic of the surface structure must corresponds precisely to previously made theoretical requirements.
A further known method for producing microstructure surface configurations is the chemical etching. However, chemical etching results in a surface configuration which is rather fissured or ragged in the zones exposed to the chemical etching. Thus, substantial notching stresses are introduced into the surface treated by chemical etching. As a result, this method is not suitable for modifying the surface configuration of structural components which have thin walls and are subject to high mechanical and thermal stresses.